A manifest turns a responsive website into an installable app. It lets users add it on their mobile phone’s home screen. When they launch the site it gets a splash screen and runs in full screen mode, basically behaving like a native app.

Caveat: For this to work HTTPS is required. Use certbot if you don’t already.

I used Manifest Generator to get started and it was easy. According to the ConFoo talk Bing indexes sites with manifest.json files and prioritizes them as smartphone compatible. A simple SEO win?